First, supporting plants actually makes them weaker. Think of a cast on your arm. Think about the atrophy when you finally take it off.
I would remove the supports for sure. Trying to see some scale in sizes here; are these plants 4' high or 6 ' high...I am very bad with metrics...??
A good point is that whoever planted these plants did a wonderful job by offsetting two lines. You will probably pull that one arborvitae...yet it will be barely noticeable. If I am understanding the scale correctly, it looks like each shrub was planted too closely to its neighbors. I am still not sure I understand the photo clearly but pulling this dying plant will surely help the look and health of the hedge itself.
Are there two lines of arborvitaes? The back one older than the front line?
What is the distance between plants?
How long ago did you fertilize and what was the product formulation?
I am also seeing a couple of other things in this photo; you've got some critter that is eating your arborvitae and I think I know who! A RABBIT. Not going to hurt unless the bunnies get so hungry they begin to eat the bark to get at the cambium beneath for calories.
Also, the soil and mulch are touching the bottom of this fence. Lovely fence but will need replacing soon if that moisture holding soil and mulch aren't removed from the wood.
We'll wait for a reply...your little arborvitae may have had its vascular system damaged by gnawing of rabbits..it is younger and more 'tender' after all than the others.
'Clay' soil has gotten such a bad rap but I have to insert some endorsement...it is wonderful soil. If you have clay soils you just need to know the management techniques. All soils are wonderful soils. The only way to amend any soil ANY soil type is to add decomposed organic matter right on the surface. All other methods are contraindicated. Talk about a lazy gardener's dream!
Just allowing your shrubs to grow without supports will allow your plants to grow straighter and stronger...that is what they want to do normally to get their foliage closer to the light. Not only straighter but with deeper thicker roots for support and thicker bark, thicker caliper of trunks and better more vigorous growth. Those supports are bad. I only use staking for mature trees that have blown over. Otherwise pruning and shaping and twine will solve your problems.
Pruning we need to discuss as that takes more explanation... but simply cutting off the edges of the top so that the entire shrub angles upward, like a bullet, not a bollard? All hedges need wider bases than the top. Wrapping a simple line of twine (green?) from the base to the top before winter will protect your plants from snow. A spiral of twine, not as tight as they do with Christmas trees for transport? We did this with 20 foot, 6 foot wide arborvitaes...not hard to do at all for a maintenance company with ladders and insurance. To replace one of those guys was not an option.
We can discuss pruning; think about removing the weight from the end of a branch or branches. That release of weight usually pops those branches back into place.
This is an idea I've used an awful lot for a screen. In the second picture do you 'see' the garbage? Your eye stops at the screen yet wind is able to flow through and your feeling of space is not compromised. And great for vines. The squares add solidity and allowing the wood to gray melds the architecture to the landscape.
My back yard with screens; later?